Switching regulators and linear regulators are well known types of voltage regulators for converting an unregulated voltage, such as a battery voltage, to a regulated DC output voltage of a desired value.
Linear regulators, also referred to as low dropout (LDO) regulators, include a control circuit and a linear switch (transistor) that is connected between the unregulated power supply and an output terminal. The control circuit monitors the regulated DC output voltage at the output terminal by way of a feedback signal, and controls the gate voltage of the linear switch such that the switch's conductance is adjusted to produce the desired DC output voltage level.
Switching regulators include a switch (transistor) and an inductor connected in series between the unregulated voltage and the output terminal, a filter capacitor, and an oscillator-based control circuit. The oscillator-based control circuit provides a time-varying control signal to the gate terminal of the switch, whereby the switch is turned on to pass the unregulated voltage to the inductor in short pulses. These pulses cause the inductor to store energy in its magnetic field, which produces the regulated DC output voltage. The filter capacitor serves to smooth and maintain the regulated DC output voltage between pulses. The regulated DC output voltage is fed back to an oscillator-based control circuit, which compares the feedback voltage with a reference voltage, and controls the oscillator output such that the switch duty cycle produces the desired regulated DC output voltage.
Switching regulators can generally be classified by the type of oscillator control as either pulse-width-modulated (PWM) or pulse-frequency-modulated (PFM) regulators. PWM regulators include an oscillator control circuit that produces a pulse train having a fixed frequency and a variable pulse width. PFM regulators, on the other hand, use oscillator control circuits that produce a fixed pulse width and a variable pulse frequency. In either case, the duty cycle of the transistor is controlled by the feedback voltage such that the regulated output voltage maintains the desired voltage level. Switching regulators are also characterized by whether they convert the unregulated voltage upward (boost or step-up configuration), convert the unregulated voltage downward (buck or step-down configuration), or invert the unregulated voltage (buck-boost or inverting configuration).
Switching regulators are generally considered to be more efficient than linear regulators, but are generally much noisier during operation. A linear regulator provides a very smooth output voltage because the linear switch is always partially on (conducting), but wastes power due to the large voltage differential across the linear switch. In contrast, the switching regulator transistor is either fully on or fully off. When a switching regulator transistor is fully on, such as in saturation or near the edge of saturation, the transistor is a highly efficient switch, and there is a minimum of wasted power through the switch. However, when load conditions suddenly increase, unlike linear voltage regulators that instantaneously adjust to the increased current demand, switching-type regulators can experience delays because the current through the inductor cannot change instantaneously with time. In addition, the control system around the switching regulator requires a longer time than a linear system to adjust the duty cycle to current-load the inductor properly.
What is needed is a voltage regulator circuit that combines the smooth, quickly responding output voltage of a linear voltage regulator and the high efficiency of a switching-type voltage regulator.